Generic Types in Visual Basic

A generic type is a single programming element that adapts to perform the same functionality for a variety of data types. When you define a generic class or procedure, you do not have to define a separate version for each data type for which you might want to perform that functionality.

An analogy is a screwdriver set with removable heads. You inspect the screw you need to turn and select the correct head for that screw (slotted, crossed, starred). Once you insert the correct head in the screwdriver handle, you perform the exact same function with the screwdriver, namely turning the screw.

Screwdriver set as a generic tool

When you define a generic type, you parameterize it with one or more data types. This allows the using code to tailor the data types to its requirements. Your code can declare several different programming elements from the generic element, each one acting on a different set of data types. But the declared elements all perform the identical logic, no matter what data types they are using.

For example, you might want to create and use a queue class that operates on a specific data type such as String. You can declare such a class from System.Collections.Generic.Queue, as the following example shows.

You can now use stringQ to work exclusively with String values. Because stringQ is specific for String instead of being generalized for Object values, you do not have late binding or type conversion. This saves execution time and reduces run-time errors.

In the preceding skeleton, t is a type parameter, that is, a placeholder for a data type that you supply when you declare the class. Elsewhere in your code, you can declare various versions of classHolder by supplying various data types for t. The following example shows two such declarations.

The preceding statements declare constructed classes, in which a specific type replaces the type parameter. This replacement is propagated throughout the code within the constructed class. The following example shows what the processNewItem procedure looks like in integerClass.

Eligible Programming Elements

You can define and use generic classes, structures, interfaces, procedures, and delegates. Note that the .NET Framework defines several generic classes, structures, and interfaces that represent commonly used generic elements. The System.Collections.Generic namespace provides dictionaries, lists, queues, and stacks. Before defining your own generic element, see if it is already available in System.Collections.Generic.

Advantages of Generic Types

A generic type serves as a basis for declaring several different programming elements, each of which operates on a specific data type. The alternatives to a generic type are:

A single type operating on the Object data type.

A set of type-specific versions of the type, each version individually coded and operating on one specific data type such as String, Integer, or a user-defined type such as customer.

A generic type has the following advantages over these alternatives:

Type Safety. Generic types enforce compile-time type checking. Types based on Object accept any data type, and you must write code to check whether an input data type is acceptable. With generic types, the compiler can catch type mismatches before run time.

Performance. Generic types do not have to box and unbox data, because each one is specialized for one data type. Operations based on Object must box input data types to convert them to Object and unbox data destined for output. Boxing and unboxing reduce performance.

Types based on Object are also late-bound, which means that accessing their members requires extra code at run time. This also reduces performance.

Code Consolidation. The code in a generic type has to be defined only once. A set of type-specific versions of a type must replicate the same code in each version, with the only difference being the specific data type for that version. With generic types, the type-specific versions are all generated from the original generic type.

Code Reuse. Code that does not depend on a particular data type can be reused with various data types if it is generic. You can often reuse it even with a data type that you did not originally predict.

IDE Support. When you use a constructed type declared from a generic type, the integrated development environment (IDE) can give you more support while you are developing your code. For example, IntelliSense™ can show you the type-specific options for an argument to a constructor or method.

Generic Algorithms. Abstract algorithms that are type-independent are good candidates for generic types. For example, a generic procedure that sorts items using the IComparable interface can be used with any data type that implements IComparable.

Constraints

Although the code in a generic type definition should be as type-independent as possible, you might need to require a certain capability of any data type supplied to your generic type. For example, if you want to compare two items for the purpose of sorting or collating, their data type must implement the IComparable interface. You can enforce this requirement by adding a constraint to the type parameter.

Example of a Constraint

The following example shows a skeleton definition of a class with a constraint that requires the type argument to implement IComparable.

If subsequent code attempts to construct a class from itemManager supplying a type that does not implement IComparable, the compiler signals an error.

Types of Constraints

Your constraint can specify the following requirements in any combination:

The type argument must implement one or more interfaces

The type argument must be of the type of, or inherit from, at most one class

The type argument must expose a parameterless constructor accessible to the code that creates objects from it

The type argument must be a reference type, or it must be a value type

If you need to impose more than one requirement, you use a comma-separated constraint list inside braces ({ }). To require an accessible constructor, you include the New (Visual Basic) keyword in the list. To require a reference type, you include the Class (Visual Basic) keyword; to require a value type, you include the Structure (Visual Basic) keyword.

Example of Multiple Constraints

The following example shows a skeleton definition of a generic class with a constraint list on the type parameter. In the code that creates an instance of this class, the type argument must implement both the IComparable and IDisposable interfaces, be a reference type, and expose an accessible parameterless constructor.

Important Terms

Generic Type. A definition of a class, structure, interface, procedure, or delegate for which you supply at least one data type when you declare it.

Type Parameter. In a generic type definition, a placeholder for a data type you supply when you declare the type.

Type Argument. A specific data type that replaces a type parameter when you declare a constructed type from a generic type.

Constraint. A condition on a type parameter that restricts the type argument you can supply for it. A constraint can require that the type argument must implement a particular interface, be or inherit from a particular class, have an accessible parameterless constructor, or be a reference type or a value type. You can combine these constraints, but you can specify at most one class.

Constructed Type. A class, structure, interface, procedure, or delegate declared from a generic type by supplying type arguments for its type parameters.